Maser emission from OH at the 6035-MHz transition

The Australia Telescope Compact Array (ATCA) has been used to measure posit ions with subarcsec accuracy for more than 50 masers at the 6035-MHz; trans ition of excited OH. Eleven of the masers are new discoveries; the remainde r complete the detailed study of masers from an earlier single-dish survey of the southern Galactic plane using the Parkes telescope. All previously k nown masers in the region were strong enough to be studied in the present o bservations, despite the common occurrence of variability. Topically, the i ntensity varies by less than a factor of two over several years, but one of the new masers is, remarkably, 100 times stronger than the upper limit mea sured six years earlier. The precise positions confirm that most of the masers coincide with a 1665- MHz groundstate OH maser counterpart that is usually stronger. However, the re are a few instances where any 1665-MHz counterpart is either much weaker or not detected. When surveyed with comparable sensitivity, we show that t he 6035-MHz masers are approximately one-third as common as 1665-MHz masers . We also add to earlier evidence that 1720-MHz masers preferentially occur at the sites hosting 6035-MHz masers. These facts are discussed in the con text of current maser pumping models. Towards most of the 6035-MHz maser sites we find a coincident 6668-MHz meth anol maser, and towards some of the maser sites we report new detections of ultracompact H II regions. Clusters of two or three sites, each showing on e, or several, of the principal molecular masing transitions, are found to be common; future comparative studies of these sites within small clusters will elucidate the precise conditions favouring different maser species. Se veral instances of absorption occur at 6035 MHz, one of them especially str iking since it extends over a broad velocity range adjacent to maser emissi on related to the ultracompact H II region 5.885 - 0.392. The absorption pr ovides new clues to understanding the geometry and excitation of 5.885 - 0. 392. Three sources were observed at 6030 NMz, an OH transition from the same exc ited state ((II3/2)-I-2, J = 5/2) as the 6035-MHz line; in one case, the ma tching velocity patterns at 6030 and 6035 MHz, their similar circular polar ization characteristics, and their spatial coincidence, reveal Zeeman split ting in a magnetic field of -9 mG, a field strength close to the upper limi t found for any such masers.